In an electrolytic cell, the input of electrical energy results in a net chemical transformation. A common feature of conventional electrolytic cells is that a substantial input of electrical energy is required to drive the electrolytic reaction at a sufficient rate. This expenditure of electrical energy reduces the efficiency of the cell.
Electrochemical cells, and in particular fuel cells, may be in the form of a membrane electrode assembly (MEA). Solid polymer electrolyte fuel cell MEAs typically have a multi-layered structure comprising (i) a Proton Exchange Membrane (PEM), (ii) a current-collecting electrode, and (iii) an electro-catalyst layer on each side. A PEM operates by virtue of containing embedded cationic sites, allowing the transmission of anions. Equally, a solid polymer electrolyte may contain fixed anionic sites, and which is capable of preferentially transmitting cations. References to PEM below are thus not exclusive.
A structure as described above is assembled from discrete elements and bonded into an MEA by the use of heat and pressure, before being assembled between gas manifolds, the whole structure being sealed against gas leakage (and cross-over) to form a single cell. The process is complex and together with the inherent cost of the PEM and the catalyst-coated carbon paper usually used as items (ii) and (iii) represent the principal costs of production of a fuel cell.
A limitation on the performance of PEM fuel cells is water management, in order that the PEM membrane remains adequately hydrated while in use. The conversion of hydrogen and oxygen to electricity yields product water which appears at the oxygen electrode. If the membrane is to remain operational, the membrane must have sufficient water-permeability to redistribute the product water and prevent local drying-out of the membrane. Drying out leads to overheating and catastrophic failure (possibly even hydrogen/oxygen crossover with the potential for explosive failure).
PEM devices operate by virtue only of the properties built into the membrane. In use as an electrolyser, the addition of water and electricity yields oxygen and hydrogen; in use as a fuel cell, hydrogen and oxygen (or air) are used, and electricity results.
Existing PEM materials, e.g. Nafion, consist of a non-cross-linked fluorinated polymer (essentially PTFE) with pendent side-chains containing an ionically active site (normally SO3). Hydrophilicity is provided by the SO3 sites. These materials must be kept hydrated with additional water (supplied via hydrated fuel gas) to operate. They are available as thin sheets, 10-30 μm thick, for assembly into cells (voltage 1V) and thus into cell stacks (typically 100 units).
A stack may be produced from individual MEAs. Since each MEA has to be produced separately, and the stack built up in series, the production of a stack is laborious.
Hydrophilic polymers, capable of having a high water content, are known. The level of water content determines their properties. Their electrical properties are defined by the properties of the hydrating solution. For example, certain hydrophilic materials such as HEMA (2-hydroxyethyl methacrylate) and MMA-VP (methyl methacrylate-vinylpyrrolidone) are well known in the biomedical field as contact lens materials, but they possess no intrinsic electrical properties. Thus, if hydrated in deionised-distilled (DD) water, the resulting polymer is a good electrical resistor but, if hydrated in an acid or alkaline solution, the material is a good conductor until the electrically active solution washes out when the hydrated polymer reverts to a non-conducting system.
U.S. Pat. No. 4,036,788 discloses anionic hydrogels obtained by copolymerisation of a heterocyclic N-vinyl monomer, a sulphonic acid-containing monomer and a cross-linking agent. Polymerisation may be conducted in the presence of a water-soluble solvent in which the monomers are soluble; the polymer is obtained in the form of an organogel from which the non-aqueous solvent is removed by distillation, evaporation or washing with water. Immersion in water causes swelling, to give a soft, pliable material that can be used to recover basic or cationic materials from an aqueous medium, or for the controlled release of such materials.
WO-A-01/49824 discloses a polymer obtainable by polymerising a sulfo group-free monomer, a sulfo group containing monomer and, optionally, a cross-linking agent. The polymers are useful for the attachment and growth of cells, and for biomedical devices and prostheses. They have a high expansion ratio.
Elements of this specification have been published before its priority date. See, for example, the Delegate Manual of the Fifth Grove Fuel Cell Symposium, Sep. 22-25 1997. These elements do not provide sufficient information for one of ordinary skill to practise the invention described below.